Field of the Invention
The invention relates to a system and method for measurement of a patient's contrast sensitivity. The system and method of this invention is particularly useful for differentiating diabetic patients with different levels of retinopathy. The system and method is also useful for detection or evaluation of other ocular diseases including, but not limited to, age-related macular degeneration, glaucoma, and multiple sclerosis.
Description of Prior Art
Approximately 250 years ago, Pierre Bouguer reported that a shadow of a vertical rod cast by a distant candle on a white screen illuminated by another nearer candle could be seen so long as the distant candle was no more than eight times as far from the screen as the nearer candle. Based on this observation, Bouguer calculated that a diminution in the illumination of one area of an image with respect to an adjacent area of 1 part in 64 could be detected with normal human vision. The measurement that Bouguer had made of what would now be called “threshold contrast” constituted the earliest quantitative measurement of visual function. In the mid-19th century, Masson, a physicist, developing new methods of photometry, noted that there was some individual variation in visual contrast threshold, and was the first to suggest that this might be of physiological and medical interest. Even then, it was not until the end of the 19th century that it was more widely recognized that contrast threshold could be raised (i.e. contrast sensitivity reduced) in various ocular conditions in which visual acuity might be no more than slightly compromised and the potential clinical usefulness of contrast sensitivity measurements began to be appreciated and noted in the text books of that period. Despite this appreciation, the clinical measurement of contrast sensitivity has to this day remained little more than a curiosity although various methods of measurement have been developed and described since that time.
There are two main reasons why the measurement of contrast sensitivity has not been more widely adopted in clinical opthalmological and optometric practice. Firstly, the major application of visual function measurement has been in optimizing the optical correction of refractive error and the adequacy of measurement of visual acuity, coupled with the simplicity of chart-based methods of determining it, has made this the measure of visual function with which everyone is familiar and the one which has become the sine qua non of opthalmological and optometric examination. In the context of refractive correction a measurement of contrast sensitivity is relevant only if acuity cannot be brought up to normal levels optically and even then may not be particularly useful except as confirmation of the existence of some underlying ocular disorder. It is rarely helpful in differential diagnosis. Moreover, to the extent that it is necessary to characterize the severity of the visual impairment that results from such disorders, the reduction in acuity compared with that of subjects with normal vision has been generally accepted as an adequate measure. Secondly, there has not, until relatively recently, been any way to measure a patient's contrast sensitivity that has been sufficiently acceptable to clinicians to encourage the adoption of this measure of visual function in routine clinical practice. This is somewhat surprising since the desirability of having a method of measuring contrast sensitivity that was as simple and reliable as that for determining visual acuity was appreciated by Bull (1882) within twenty years of the first publication of the standardized letter charts (Snellen, 1862) that had originally prompted the widespread use of such charts for clinical measurement of visual acuity. Bull proposed that contrast sensitivity should be measured using a chart with letters all of the same large size whose contrast with respect to their white background decreased progressively from the top to the bottom of the chart, the letters being black at the top of the chart and then becoming fainter and fainter on successive lines until those at the bottom were invisible. This chart would be used in much the same way as an acuity chart, the contrast threshold being recorded as the contrast of the faintest letters that could be correctly named. It was a good idea but, unfortunately, Bull was unable to make such a chart.
In 1988, Pelli, Robson and Wilkins designed and manufactured such a letter contrast sensitivity chart, see FIG. 1, using the latest technical advances in printing methods. These letter charts provided a more practicable way of measuring contrast sensitivity in the clinic than laboratory-style systems based on electronic display devices and have undoubtedly increased interest in the clinical application of contrast sensitivity measurement, particularly in the assessment of low vision. However, it is still seems that although numerous studies have shown that reduction in contrast sensitivity can provide a useful measure of disease progression, such measurements are not routinely made to aid the management and treatment of progressive diseases in opthalmological practice. The reason for this is perhaps to be sought in the fact that clinicians working in a clinic will normally assess a patient's disease status using funduscopic and tomographic methods and would gain little from any measurement of visual function.
However, there are other circumstances in which it may be more appropriate to consider making measurements of visual function as an aid to management of disease such as, but not limited to, age-related macular degeneration (AMD) and diabetes. One known chart is the Amsler grid that is used for monitoring the progression of AMD and to provide information related to other diseases, such as diabetes.